ART: Forward-declare Signature in art_method.h (iwyu)

Test: mmma art
Change-Id: I16e2083db5b52c9373350a1072550d1e83355ab5
diff --git a/runtime/art_method-inl.h b/runtime/art_method-inl.h
index c25bd00..61c261c 100644
--- a/runtime/art_method-inl.h
+++ b/runtime/art_method-inl.h
@@ -29,6 +29,7 @@
 #include "dex/dex_file_types.h"
 #include "dex/invoke_type.h"
 #include "dex/primitive.h"
+#include "dex/signature.h"
 #include "gc_root-inl.h"
 #include "imtable-inl.h"
 #include "intrinsics_enum.h"